[PATCH 4 of 5] cext: fix a warning about differing const qualifiers on Windows

[Yuya Nishihara]

On Fri, 10 Aug 2018 01:01:18 -0400, Matt Harbison wrote:
> # HG changeset patch
> # User Matt Harbison <matt_harbison at yahoo.com>
> # Date 1533875287 14400
> #      Fri Aug 10 00:28:07 2018 -0400
> # Node ID 051a9598e4dbec47aa69c9ba10cd0f488828e9c1
> # Parent  eb3d685ec313b5d5bca7824e5e3e577509a486de
> cext: fix a warning about differing const qualifiers on Windows

Queued 2-4, thanks.